Antireflection coatings are used for a variety of substrates, particularly glass. Broad band antireflection coatings are typically designed to minimize reflection throughout the visible range of the spectrum. There are many ways to apply such coatings to glass, including dip coating using sol gel techniques and more costly vacuum techniques such as sputtering.
Safety glass for automobiles, picture frames, display cases and the like is typically formed by subjecting cut or shaped glass to a high temperature heat treatment, followed by rapid cooling. The process induces stress in the glass which thereby contributes to enhancing the mechanical stability of the glass forming safety glass. Such glass, due to its enhanced properties, cannot be cut, trimmed or otherwise mechanically treated after tempering. As such, glass substrates are cut, shaped and/or trimmed prior to tempering.
It is advantageous in safety glass manufacture to provide coatings to minimize glare and the like. However, many difficulties are encountered in applying antireflection or other optical coatings to glass which is to be tempered. In the prior art, such coatings cannot be practically applied prior to tempering because the tempering process creates disadvantages with respect to resulting optical properties. During tempering, thinning of the coating layers occurs, typically with respect to the outer layer of a multilayer antireflection coating. The outer layer may be burned off and/or the entire coating system distorted. Further, the index of refraction may be affected due to changes in the crystal structure or density changes of some materials during tempering. Such changes affect the optical properties of the whole system. Titanium dioxides, which are commonly used as a middle layer in three layer antireflection coatings, are significantly affected by tempering.
In addition to the above-noted problems, the quick cooling used in the tempering process which induces the desired stress within the glass, unfortunately also induces undesirable stress into the antireflection or other optical coating subjected to tempering. The stress in the coating, however, is not beneficial and often leads to disintegration, cracks or microcracks. The coating will appear hazy as a result, or may be completely destroyed such that it cracks or flakes off.
Because of the disadvantageous results of tempering coated glass, antireflection coatings have been applied using various coating techniques after the glass has been tempered. Unfortunately, this means that large pieces of commercial glass must first be cut and shaped, then tempered. As a result, coating is done on smaller, pre-cut pieces of tempered glass. This process is time-consuming, inefficient, and, therefore, tends to be uneconomical.
In attempting to coat already cut tempered glass with antireflection coatings, sophisticated vacuum coating techniques are typically required, because practical dip-coating techniques used for standard antireflection coated glass have not been used successfully in the art of making tempered glass. Application of an antireflection coating using dip coating techniques involves heat treatments to form the coatings which cause relief in the stress created by tempering the glass. Relief of the stress resulting from tempering contributes to deteriorating the mechanical properties provided by tempering. While there has been an attempt to form tempered glass using dip coating which involves estimating the changes in optical properties and trying to compensate for them prior to heat treatment, such methods lack sufficient quality control and do not maintain adequate reflection color. As such, dip coating, while cost effective and practical for use in coating standard glass, has not been practically and effectively used for coating tempered glass or other heat treated inorganic substrates to provide coatings of adequate reflection color, and of reproducible optical quality.
As a result of the inability to use dip-coating techniques, it has been necessary to use techniques such as cathodic sputtering to apply antireflection coatings for tempered glass. Examples of such techniques are described in U.S. Pat. Nos. 5,059,295 and 5,028,759 of Finley.
While sputtering allows for application of antireflection coatings for making tempered glass, there is still a need in the art for an economical process for large scale formation of antireflection coated glass which can be cut and used to form tempered glass. There is further a need in the art for an economical method for coating glass prior to cutting and trimming for tempering such that larger pieces of glass may be coated while still providing tempered glass of optical quality.
The invention includes a multilayer antireflection coating for use in coating a heat treatable inorganic substrate comprising an inner layer for contact with an inorganic substrate having at least two different metallic oxides and being capable of providing an index of refraction of from about 1.54 to about 1.90 after curing. The coating further includes a middle layer on the inner layer comprising an oxide of zirconium and being capable of providing an index of refraction of at least about 1.90 after curing and an outer layer on the middle layer comprising at least one metallic oxide and being capable of providing an index of refraction of about 1.54 or less after curing. The optical properties of the multilayer antireflection coating are substantially retained after the multilayer antireflection coating is applied to an inorganic substrate and subsequently subjected to a heat treatment
The invention further includes a bilayer antireflection coating for use in coating a heat treatable inorganic substrate comprising an inner layer for contact with an inorganic substrate comprising an oxide of zirconium and at least one oxide of a metal different from zirconium, wherein the inner layer is capable of providing an index of refraction of from about 1.54 to about 1.90 after curing, and an outer layer on the inner layer having at least one metallic oxide and being capable of providing an index of refraction of about 1.54 or less after curing. The optical properties of the bilayer antireflection coating are substantially retained after the bilayer antireflection coating is applied to an inorganic substrate and subsequently subjected to a heat treatment.
Also within the invention is a multilayer antireflection coating for use in coating a heat treatable inorganic substrate comprising an inner layer comprising an oxide of zirconium and being capable of providing an index of refraction of at least about 1.90 after curing; a first middle layer on the inner layer having at least one metallic oxide and being capable of providing an index of refraction of about 1.54 or less after curing; a second middle layer on the first middle layer comprising an oxide of zirconium and being capable of providing an index of refraction of at least about 1.90 after curing; and (d) an outer layer on the second middle layer having at least one metallic oxide and being capable of providing an index of refraction of about 1.54 or less after curing. The optical properties of the multilayer antireflection coating are substantially retained after the multilayer antireflection coating is applied to an inorganic substrate and subsequently subjected to a heat treatment.
A method for making a heat treated antireflective coated inorganic substrate is also provided. The method comprises coating an inorganic substrate with an inner layer comprising a mixture of at least one first oxide of a metal selected from the group consisting of titanium, zirconium, lanthanum, tantalum, and niobium and at least one second oxide of a metal selected from the group consisting of silicon, and aluminum to form an inner layer. The inner layer is coated with a middle layer comprising an oxide of zirconium. The middle layer is coated with an outer layer of at least one oxide of a metal selected from the group consisting of silicon, and aluminum; and the coated inorganic substrate is heat treated.